Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations only declare destructiveHint=true; the description adds the critical economics (charges the account, real mail), the test-key escape hatch, and the cancellation window ('Cancelable only before send_date'). These are exactly the behavioral traits the hint cannot convey.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The money warning is front-loaded in caps, followed by tightly packed parameter facts and the underlying endpoint. No sentence is wasted.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a 9-parameter mutation with nested objects and no output schema, the description covers cost, cancellation, and input shapes well. It omits any mention of what is returned (e.g. the letter id used later with lob_get_letter/lob_cancel_postcard-style flows), a minor gap given no output schema.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the schema already documents every parameter. The description largely restates it (adr_/inline address forms, HTML/URL/tmpl_ for file, color required) rather than adding new syntax or defaults, so the baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('SENDS A REAL LETTER', 'POST /v1/letters') and makes the scope unmistakable. It is trivially distinguishable from siblings like lob_create_postcard and lob_create_self_mailer, which are different physical products.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives actionable guidance: use a test_ key to avoid real mail/charges, and the letter is cancelable only before send_date. It does not explicitly name an alternative (e.g. postcards) or state when a letter is preferred over one, but the context for correct invocation is clear.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.